Yet another plug reading question

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May 11, 2007
I was reading over the two articles on the DragStuff website ( ... plugs.html and ... plugs.html) and even they don’t seem to agree on how to read jetting, unless I’m reading it wrong. The first one specifically says not to use the porcelain to read jetting and to use the base ring for that. The other says that different carb circuits will show up on different areas on the porcelain and that the base ring is only an indication of whether you tightened your plugs enough. There are other differences as well but right now I’m mostly concerned with jetting so that’s what really caught my attention.
So… what’s the general consensus with everyone here? I tend to side with the first article and I remember reading that Gordon Jennings article that said much of the same. In particular if the plugs are running hot enough to burn off deposits and keep itself from fouling then the only place where there should be soot is at the very bottom of the insulator where things cool down a bit. I know this may spark another debate on the issue but I couldn’t help but ask since the two articles seem so different.
It can be a little confusing because they are Drag guys and don't care too much about idle setting. But the base info is all good. Yes all the idle and low end stuff is read off the base ring. Plug heat range off the ground strap and main jet is read at the very base of the porcelain using full throttle openings say third gear up hill if you can.
dredging up some ancient history thread here..
My plugs run cream to off white color. At all speeds. Unfortunately, plug coloration is way different than yesteryear, with leaded fuel. Unleaded fuel only colors when GROSSLY over rich. Other operational observations are used to verify proper mixture. Fuel economy where you expect it? 40 something? Runs well with no flat spot, misfire? Runs well when cold temps arrive? If yes, then you're not too lean. I see people trying to tune plug color to coffee/cardboard color.. and they end up Hog rich. Envision washing the cylinders down.
What you read on the plug has as much to do with how you do a plug chop as it does with carbeuration (including main, needle, slide and idle circuits).
There should be about a 2mm wide black ring at the bottom of the insulator right down where it joins the metal. If you are racing seriously, it should just disappear as you reduce main jet size. The bike will always go fastest just before the motor destroys itself. The jetting at 3/4 throttle is much more important than the effect of running slightly rich on the mains. If the motor coughs as you ride the bike around the tight twisty stuff, you need to raise the needles until the cough just disappears. I use Mk2 Amal carburettors but always multi-tapered Mikuni needles, even with alcohol. It is important to run as lean as possible right down the needle, for maximum performance. If the motor coughs as you use the throttle, you have gone too far.
If you are using alcohol, it is possible to have the situation where the tip of the needle obstructs, and increasing the main jet has no effect in richening the high speed mixture. It may be necessary to recess the top of the needle jet, so that metering by the needle does not occur at full throttle. You must make sure that the tip of the needle always locates in the needle jet - if it comes out, could result in a high speed crash !
Hijacking the plug question somewhat, have any of you used a Gunson Gastester for setting mixture?
powerdoc said:
Hijacking the plug question somewhat, have any of you used a Gunson Gastester for setting mixture?

This brings up an interesting question. If you are tuned correctly (regardless of how one determines it), how do you determine the correct plug heat range. I know this is stepping out beyond setting up a more or less stock Norton twin but if you determine jetting on a dyno with a gas analyzer, how do you go about selecting the correct plug heat range?
The black ring should be there at the bottom of the porcelain where it meets the metal. If you are looking for max performance, the ring can just disappear, but if you go that far you are in the danger zone. Two strokes in particular often run that lean, they have to to go really fast, but it is not a nice thing to be doing. If you see beads of aluminium on the plug you are lucky, the motor can destroy itself if you even sniff. I've never used a gas analyser to set jetting, I don't believe that is an accurate way of doing it. ( I am actually an industrial chemist, - the gas get when you burn a fuel might be the theoretical result from the stochiometric mixture, but not give maximum power) . The way to get it right is by lowering the needles until the motor coughs, then raising them one notch. Mikuni needles have three tapers, so you can get a lot of adjustment right through the full range of throttle openings, by swapping needles. It is important that the bike accelerates cleanly and with really good power at all throttle openings. Setting the mains is less important, especially with alcohol. If you are riding the bike on full throttle for extended periods, you are in a strange situation anyway. It is easy to do a plug chop and check that you are safe. It is possible to jet using a warmer plug, then change it to the next colder and get the ring back on the porcelain. For normal use, it should always be there. If you are racing the bike, methanol is a much better fuel than petrol to be using - it hides up the tuning errors, and if it is rich the motor won't slow nearly as much as if you run petrol rich.
The reason that Australian riders were so fast in Europe in the fifties and sixties was that in Australia we used alcohol fuel, and were used to the performance. The Brits could get their manx Nortons going as fast as ours using petrol. Getting jetting right is all a matter of patience and being systematic, and you need to remember that a faulty ignition system can totally stuff up the exercise. If you are using methanol, it is normal to advance the ignition timing by about four degrees from the manufacturers specified petrol timing.

Have fun and don't destroy your motor, make the changes carefully and systematically, and always look for max power. If the weather turns really cold, check that you still have the black ring on the porcelain of the plug. If it is there, you are still safe.
When I built my Commando 850, I bought two 34mm Mk2 Amal carbs from Mick Hemmings fitted with the alcohol kit. The needle jets were 0.120 inch, and the mains about 800 amal. From my previous experience, I knew that was miles too rich. So I bought some brass hex and made a pair of needle jets using number drills at 0.117 inch. I fitted 670 amal main jets. I threw away the amal 'Y' needles and fitted Mikuni 6DP6 needles, normally used for petrol (same diameter at the parallel part). I rode the bike around our local circuit and lowered the needles until the motor coughed as I rode it around the tight stuff, then raised the needles one notch. The bike immediately accelerated cleanly with really good power. I've checked the plugs to make sure that the main jets are not silly, but I haven't bothered to optimise. Getting jetting right using methanol is not rocket science. I would never use petrol for racing, it is all too hard.
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